1. Field of the Invention
The present invention relates to a system for controlling a steering angle of rear front wheels of a rear two-axle vehicle, such as a truck, having a rear front axle and a rear rear axle.
2. Description of Related Art
Conventionally, there has been disclosed a steering angle ratio control system as a system for steering rear wheels, which is configured so that a rear wheel steering angle corresponding to a steering wheel angle is controlled by an electro-hydraulic control circuit in relation to a vehicle speed, the electro-hydraulic control circuit increases the steering angle ratio of rear wheel to front wheel at the time of low speed and decreases the steering angle ratio with increasing vehicle speed, and further makes the steering angle ratio zero when the vehicle speed is higher than a predetermined value (Unexamined Japanese Patent Publication 1990-124381) (hereinafter referred to as a first conventional example). In this system, the rotation of an input shaft rotated in relation to an output shaft of a front wheel steering mechanism is transmitted to a differential control valve via a steering angle ratio control mechanism in relation to the vehicle speed, and a hydraulic circuit to a rear wheel steering actuator is controlled by the differential control valve. Also, the rear wheels are steered by the rear wheel steering actuator, and the steering angle ratio control mechanism is controlled by the electro-hydraulic circuit. The steering angle ratio control mechanism, which is provided on the input shaft, has a protrusion which rotates in response to the steering wheel angle and a notch which is formed in a notch member provided at the distal end of a drive shaft of the differential control valve and engages with the protrusion with a gap formed in the peripheral direction.
In the steering angle ratio control system configured as described above, the electro-hydraulic control circuit regulates the magnitude of the gap between the protrusion and the notch in relation to the vehicle speed, that is, it controls the width of a dead zone (play) from the neutral position of a steering wheel until the turning of the drive shaft is conveyed, in relation to the vehicle speed.
Therefore, the steering angle ratio of the rear wheel steering angle to the front wheel steering angle decreases gradually as the vehicle speed increases. As a result, in low-speed running, the rear wheels are steered relatively greatly in antiphase with the front wheels, so that small sharp turning property is exhibited. Also, in medium-speed running, the rear wheels are scarcely steered in the ordinary range of steering wheel angle, and in high-speed running, the protrusion is incapable of engaging with the notch, so that the rear wheels are not steered at all, by which the straight-line running property is improved.
On the other hand, there has been disclosed a rear front axle wheel steering system for a rear two-axle vehicle (Japanese Utility Model Publication 1994-21818) (hereinafter referred to as a second conventional example). The system is configured so that rear front wheels can be steered in phase with the front wheels in relation to the front wheel steering, the steering angle of the rear front wheel is controlled slightly more than the steering angle of the front wheel in the range in which the steering angle of the front wheel is small, and the steering angle of the rear front wheel is controlled slightly less than the steering angle of the front wheel in the range in which the steering angle of the front wheel is large. In this steering system, a pitman arm for the front wheel steering system is connected to a front wheel knuckle arm via a drag link, and the drag link is connected to a rear wheel knuckle arm via a relay rod. The rear wheel knuckle arm is provided with an elongated groove, and the relay rod is slidably connected to the elongated groove via a pin. Also, the elongated groove has a first guide groove for controlling the rear front wheels with a slightly larger steering angle than the front wheels, a pair of second guide grooves provided in front and rear of the first guide groove to control the rear front wheels with a slightly smaller steering angle than the front wheel, and a pair of inclined intermediate groove connecting the first guide groove to the second guide grooves.
In the steering system configured as described above, a lever ratio of the lever length of the rear wheel knuckle arm to the lever length of the front wheel knuckle arm is changed according to the magnitude of the front wheel steering angle. As a result, at the time of high-speed running when the steering angle is small, the rear front wheels are made in phase with the front wheels and the steering angle thereof is made slightly larger than the steering angle of the front wheel, whereby the steering stability can be improved. Also, at the time of low-speed running when the steering angle is large, the rear front wheels are made in phase with the front wheels and the steering angle thereof is made slightly smaller than the steering angle of the front wheel, so that the rear front wheels are rotated smoothly in turning.
However, in the case where the first conventional example is applied to the second conventional example, that is, the configuration is such that on a rear two-axle vehicle, the steering angle ratio of rear front wheel to front wheel is made high at the time of low speed, the steering angle ratio is decreased as the vehicle speed increases, and further when the vehicle speed is not lower than a predetermined value, the steering angle ratio is made zero, the rear wheels are steered even if the vehicle speed is zero. Therefore, for example, if a difference in steering angle between the target steering angle and the actually-measured steering angle of the rear front wheel is large, the vehicle cannot be started smoothly. Also, if an attempt is made to decrease the difference in steering angle between the target steering angle and the actually-measured steering angle of the rear front wheel when the vehicle speed is zero, a large steering force for the rear front wheels is needed, so that an actuator for steering the rear front wheels must be made large in size.
An object of the present invention is to provide a rear front wheel steering control system for a rear two-axle vehicle, which can steer rear front wheels by means of relatively small sized rear front wheel steering means, and thereby can start a vehicle smoothly.
The present invention provides a rear front wheel steering control system for a rear two-axle vehicle, comprising front wheel steering means driven by an engine for steering front wheels; rear front wheel steering means driven by the engine for steering rear front wheels; a front steering angle sensor for detecting a steering angle xcex4f of the front wheel; a vehicle speed sensor for detecting a vehicle speed V; and a controller for controlling the engine and for controlling the rear front wheel steering means so as to steer the rear front wheels in the same direction as that of the front wheels based on the detection outputs of the front steering angle sensor and the vehicle speed sensor, wherein the rear front wheel steering control system further comprises a rear steering angle sensor for detecting a steering angle xcex4r of the rear front wheel; and when the vehicle speed sensor detects a vehicle speed V lower than a first reference value V0, and a difference in steering angle between a target steering angle xcex4p of the rear front wheel based on the steering angle xcex4f of the front wheel calculated by the controller and an actually-measured steering angle xcex4r of the rear front wheel detected by the rear steering angle sensor is not smaller than a second reference value xcex40, the controller increases the rotational speed N of the engine to a third reference value N0; and when the difference in steering angle becomes smaller than the second reference value xcex40 or when a predetermined time T0 has elapsed, the controller returns the rotational speed N of the engine to the original value.
According to the present invention, when the vehicle is stopped, the actually-measured steering angle xcex4r of the rear front wheel is sometimes different greatly from the target steering angle xcex4p thereof. If the engine is restarted in this state, the controller reads the detection outputs of the front steering angle sensor, the rear steering angle sensor, and the vehicle speed sensor, and then calculates the target steering angle xcex4p of the rear front wheel with respect to the steering angle xcex4f of the front wheel. At this time, the vehicle is stopped in an idling state, so that the controller judges from the detection output of the vehicle speed sensor that the vehicle speed V is lower than the first reference value V0. Next, the controller calculates the difference in steering angle between the target steering angle xcex4p and the actually-measured steering angle xcex4r of the rear front wheel, compares the difference in steering angle with the second reference value xcex40, and judges that the difference in steering angle is not smaller than the second reference value xcex40. Therefore, the controller increases the rotational speed of the engine to the third reference value N0, and then actuates the rear front wheel steering means for steering the rear front wheels. When the difference in steering angle between the target steering angle xcex4p and the actually-measured steering angle xcex4r of the rear front wheel becomes smaller than the second reference value xcex40 or when the rear front wheel steering means is actuated and the predetermined time T0 has elapsed, the rear front wheel steering means is stopped, and thereafter the rotational speed of the engine is returned to the original value.